OBJECTIVETo study the retentive force and deformation of acetal resin clasp.

METHODS40 premolars and 40 molars were cast respectively. Undercut of 0.25 mm or 0.50 mm depth were measured for each with undercut gage. According to the type of abutment and the depth of undercut, the specimens were divided into 4 groups: Premolars with 0.25 mm undercut, premolars with 0.50 mm undercut, molars with 0.25 mm undercut and molars with 0.50 mm undercut, 20 specimens each group. 10 three-arm clasps with resin and Co-Cr alloy were fabricated in each group, respectively. The clasps were set into the corresponding abutments and soaked in distilled water. The retentive force of the clasps when 0, 720, 1440, 2160, 2880, 3600, 4320 consecutive times of setting in and removing out from the abutments were measured. The distance between the tips of retentive arm and resistant arm after 0 and 4320 cycles were recorded.

RESULTS1) The mean retentive force of resin clasps (1.69 N) was significantly lower than that of Co-Cr clasps (5.87 N) (P<0.01). With the same factors, the retentive force of resin clasps were significantly less than that of Co-Cr clasps (P<0.01). The retentive force of molar clasps were significantly lower than that of premolar models (P<0.01). The retentive force of 0.25 mm undercut clasps were significantly lower than that of 0.50 mm undercut clasps (P<0.01). With increasing time of the cycles, the retentive force of Co-Cr clasps significantly reduced (P<0.01), but the retentive force of resin clasps didn't change significantly (P>0.05). 2) After 4320 times, the distance between the tips of retentive arm and resistant arm of Co-Cr clasps increased significantly (P<0.05), but the distance between the tips of resin clasps didn't change significantly (P>0.05).

CONCLUSIONThe retentive force and deformation of the resin clasp are significantly lower than those of Co-Cr clasp.